The use of X-rays for imaging is well known. Typically an X-ray source and detector are placed on opposing sides of the object to be imaged and the X-ray source is activated. X-rays passing through the object are detected and formed into an image.
The detection of X-rays may be accomplished using any of a number of methods. Under one method, X-ray film may be used as the X-ray detection device. In this case, a two-dimensional image may be formed by simply developing the image formed on the film by the X-rays passing through the object of interest.
Alternatively, the detection of X-rays may be accomplished with a one- or two-dimensional array of scintillating elements. As is known, a scintillating element functions to absorb X-ray energy and re-emit the energy in the form of lower-energy photons, typically in the visible light range. A photodetector is usually attached to an outer surface of the scintillator to detect the light produced by the interaction of the X-rays with the scintillator. The magnitude of the electrical signal from the photodetector (which is a function of the flux of X-rays that hit the detector element) is used to represent the detected X-ray signal.
The use of portable X-ray inspection systems for trucks is generally known. Such systems are typically used to perform non-invasive inspection of trucks for contraband (e.g., explosives, drugs, etc.). Often an X-ray beam is directed through the truck to a set of detectors on an opposing side.
As the radiation of the X-ray beam passes through the truck, the contents of the truck attenuate the beam based upon the density of the contents. Based upon the attenuation, an image may be formed of the truck's contents. By comparing a truck's manifest with the X-ray image, law-enforcement personnel may make a determination of whether on not they have probable cause to believe that any laws have been broken.
While portable X-ray imaging systems for trucks work well, they are difficult to use in some cases. For example, vehicle imaging systems are subject to a great deal of variation in signal levels. If the output of the X-ray source is adjusted to keep the signal passing through low density areas (e.g., air paths or with negligible material in the path of the beam) below the maximum level that the detector system is able to process, then the signal passing through other regions of the object (with a high density) may be too low to measure variations in that low-level signal. Without prior knowledge of the distribution of material in the object to be imaged, the power level setting of the X-ray source (and the detector full-scale level) is chosen based upon experience. If that setting is wrong, the process may need to be repeated. Because of the importance of vehicle inspection, a need exists for a better method of producing images from X-rays passing through vehicles.